This invention relates to an unsaturated polyester resin composition for use, in the manufacture of, for example, electrical laminates. The term "electrical laminates" as used herein refers to unclad insulation boards for mounting various electrical and electronic parts thereon as well as one or both sided metal clad laminates for use in the manufacture of printed circuit boards (PCBs).
Electrical laminates made from unsaturated polyester resins are now commercially available and consumed in large quantities. These laminates are produced by a continuous process as disclosed in, for example, U.S. Pat. No. 4,372,800 assigned to the assignee of this application. Unsaturated polyester resins in general find a wide variety of uses owing to their excellent mechanical properties and molding workability with various reinforcing materials. Needs exist for resins having an improved impact strength without compromising their heat resistance in the field of large FRP products. For use in the manufacture of PCBs they must comply with high standard requirements for properties in terms of punching property, solder dip resistance, impact strength and the like. These property requirements have become severer when PCBs are processed on automated basis and densely packaged with parts.
Attempts have been made to improve the impact strength by modifying unsaturated polyester resins with flexible resins, plasticizers, liquid butadiene based polymers, cross-linked diene- or acrylate-based rubbers and the like. However, the incorporation of these modifiers is not satisfactory in that they tend to compromise heat resistance, substrate-impregnating property, stability upon storage without phase separation and other properties of the resulting composition.
It is, therefore, a major object of the present invention to overcome the above-discussed problems. Other objects and advantages thereof will become apparent as the description proceeds.